Injection molding of plasticized synthetic resin, conducted by the use of an injection molder has a problem in that it is difficult to keep the quality of molded products constant by making up the shrinkage of the plasticized synthetic resin in cooling and setting in the cavity of the mold. To solve the problem, various control methods for the injection process have been proposed.
The art disclosed in Japanese Examined Patent Publication No. Hei. 1-33331 (1989) is an example of an attempted solution to the above-mentioned problem, wherein a screw or a plunger moved forward at a high speed in the material filling process is slowed during the dwelling process to apply dwelling pressure to the plasticized synthetic resin placed inside the cavity, then, the screw or the plunger is locked in movement and held in place until the plasticized synthetic resin inside the gate of the mold is cooled and solidified(gate-sealed) to prevent further of the plasticized synthetic resin between the inside of cylinder and the cavity.
However, the gate-sealing is a function occurring inside the mold, so that it is quite difficult to detect precisely when gate sealing occurs. Furthermore, the time when the gate-sealing occurs varies depending on subtle changes of temperatures of the mold and the plasticized synthetic resin, so that the time the screw or the plunger is locked must be set excessively long to ensure and completion of the gate sealing. Also, locking of the screw or the plunger needs a feed-back control device, resulting in requiring an expensive injection molder. Hence, it further leads to a problem of a low productivity due to a longer locking time and a high cost of production due to the use of a complicated control device.
Also, to compensate for the shrinkage of the plasticized synthetic resin cooled and set in the cavity, the conventional control method as above-mentioned supplies a corresponding volumetric amount of material to the shrinkage from the molded product itself by locking the screw or the plunger until the gate is sealed in the dwelling process to prevent a flow of the plasticized synthetic resin in the gate so as to release the pressure of the resin which has been compressed by the filling operation. By this method, the shrinkage can be made up and a molded product of a fixed volume corresponding to an internal volume of the cavity can be provided. But, as long as the plasticized synthetic resin has a compressibility, its density changes depending on the pressure and temperatures, so that it is hard to keep a constant weight of a molded products.
The present invention has been designed to overcome the above-mentioned problems. An object of the invention is to provide a molding cycle control method of an injection molder which method can shorten the time required for each molding cycle without using a complicated device yet keep the weight of the molded products constant.